Monoamine oxidase (MAO, EC 1.4.3.4) is a flavin dependent metabolic enzyme responsible for the oxidative deamination of both endogenous, aminergic neurotransmitters and xenobiotic amines. There are two reported isoforms of MAO, MAO-A and MAO-B, which arise from two independent genes (Bach, et. al., Proc. Natl. Acad. Sci., 1988, 85, 4934-4938). Both forms of MAO are distributed in a variety of tissues in varying amounts throughout the body; in the human brain, MAO-B is present to a greater extent then MAO-A (Saura, et. al., Neuroscience, 1996, 70, 755-774).
MAO-A has greater selectivity for serotonin and adrenalin while MAO-B is selective for tyramine and phenethyl amine while both isoforms will metabolize dopamine. Studies have shown that the level of MAO-B activity in the brain increases with age (Fowler, et. al., J. Neural Transm., 1980, 49, 1-20). The process of oxidative deamination, which produces both peroxide and aldehydes as byproducts, has also been associated with an increase in oxidative damage in the brain, especially to dopaminergic neurons, potentially exacerbating the neuronal degeneration associated with diseases such as Alzheimer's Disease and Parkinson's Disease. There are also reports that the level of MAO-B activity present is greater in patients with Alzheimer's disease which may be linked to the increased cognitive impairment of Alzheimer patients (Dostert, et. al, Biochem. Pharmacol., 1989, 38, 555-561; and Emilsson, et. al., Neuroscience Letters, 2002, 326, 56-60). This link between oxidative stress and progression of neuronal damage suggests that inhibition of MAO-B will minimize the degenerative effects of both of these diseases, presumably by preventing the metabolism of monoamines in the brain. Furthermore, the relative increase in dopamine levels, due to inhibition of its metabolism, may have effects on downstream regulation of plasticity-associated cognitive function, which may help repair, not just impede the progression of these diseases.
The use of selective MAO-B inhibitors for neurological diseases has been known for some time (Bentue-Ferrer, et. al., CNS Drugs, 1996, 6, 217-236). Most early MAO inhibitors for the treatment of depression were irreversible inhibitors with minimal selectivity for MAO-B versus MAO-A. This can be problematic due to potential side effects associated with both the subsequent inability of the irreversibly inhibited enzyme to effectively metabolize dietary amines associated with cardiovascular events (the “cheese effect”) and the potential for drug-drug interactions with other drugs that are metabolized by MAO-B. More recent drugs, including selegiline and rasagiline, while still irreversible inhibitors, have greater selectivity for MAO-B, and have better side-effect profiles (Chen & Swope, J Clin Pharmacol. 2005 45, 878-94). There is currently a need for compounds that are useful for enhancing cognitive function and for treating cognitive deterioration in Parkinson's Disease and Alzheimer's Disease, as well as compounds that can generally improve cognition in normal, diseased, and aging subjects. Preferably, such agents will have higher potency and/or fewer side-effects than current therapies.